Since a ceasefire collapsed this week, at least 180 people have been killed in rebel-held areas of Aleppo province, including the besieged eastern part of the city, the Syrian Observatory for Human Rights said Saturday.
The area is facing a heavy barrage of airstrikes by the Russian and Syrian air forces, activists say, noting that hospitals are overwhelmed by the violence and undersupplied due to a siege imposed on Aleppo since July.
"We do not have enough doctors to cope with such a high number of casualties," said Ibrahim al-Hajj, a volunteer with the White Helmets, a rescue group that operates in rebel-held districts. He said more than 200 injured people are in makeshift hospitals.
Photos and videos released by activists showed injured people lying on floors due to a lack of beds, covered in blood.
Three out of four stations belonging to the White Helmets were hit by strikes this week, while hospitals have been repeatedly damaged and destroyed over the course of the war, which began in 2011.
Among the dead since the truce broke down on Monday night are 26 children, the Britain-based monitoring group reported, amid fresh airstrikes and an advance by pro-government troops, tightening their grip on the ground to enforce the siege.
Water supplies across the city, including to the government-held west, have been badly damaged, leaving some 2 million people in a dire situation.
Syrian government forces this week announced plans to launch a ground invasion on the eastern districts, where the UN estimates some 250,000 people still live. Residents in the rebel-held zones say there is no safe way for them to flee the besieged area.
The latest government advance, taking the Handarat Camp and other areas in northern Aleppo, puts further pressure on rebels and are the first signs of gains on the ground by President Bashar al-Assad's forces since Thursday, when the offensive began.
"The shelling and the raids did not stop. It is continuous," Bahaa al-Halabi, an activist in eastern Aleppo, told dpa.
Mahmoud Rslan, who works as a cameraman in the area, said the raids included artillery and barrel bombs. Rescue teams were unable to reach areas where civilians were believed to be stuck under rubble.
International efforts to enforce a ceasefire have failed and there appears to be no ongoing political track that will bring about any immediate cessation of hostilities.
Moreover, many besieged areas, with hundreds of thousands of people, remain without aid.
The United Nations Security Council is scheduled to meet on Sunday at 11 am (1500 GMT) to discuss the situation in Aleppo.
UN Secretary General Ban Ki-moon was "appalled by the chilling military escalation" in Aleppo, noting that it was facing the "most sustained and intense bombardment" since the Syrian crisis began in 2011, said Stephane Dujarric, Ban's spokesman.
European foreign policy chiefs meeting with US Secretary of State John Kerry in Boston said Russia must prove it is willing to salvage the ceasefire in Syria.
"The devastating events in Syria this week underscore what we have been saying for some time: the burden is on Russia to prove it is willing and able to take extraordinary steps to salvage diplomatic efforts to restore a cessation of hostilities, allow unfettered humanitarian assistance and create the conditions necessary for the resumption of UN-led talks about a political transition," the foreign ministers of France, Italy, Germany, Britain and the US and the European Union's foreign policy chief said.
Before the meeting, Kerry called developments in Aleppo "unacceptable" and called on Russia to pressure the Syrian regime to end its assault.
Humanitarian deliveries have been further hampered by an attack Monday on an aid convoy outside Aleppo, which killed 21 people. The US blames Russia, but Moscow denies.
However, 36 trucks from the international Red Cross entered al-Waer, a district in central Homs province. This week, hundreds of rebels and their families evacuated the area, under a deal.